Hard disks are required to have high capacity and to be produced with reduced cost. Under such circumstances, glass substrates are required to have high smoothness and cleanliness as well as to have high productivity. In order to achieve high smoothness, a method is known in which a colloidal silica polishing agent is used for a final polishing step. As a means to improve the productivity at the same time, Patent Document 1, for example, discloses a polishing machine in which polishing liquid containing colloidal silica is circulated.    Patent Document 1: Japanese Patent Application Laid-open No. 2008-246645